1. Field of the Invention
This invention relates to testing apparatus, and, more particularly to apparatus used for determining the various characteristics of electrical motors during starting and running.
2. Description of the Prior Art.
Within the past two decades, the field of electrical power has undergone several remarkable changes. The development and use of integrated circuits led to the construction of systems such as minicomputers which are becoming less expensive, occupy little space, and are controlling many tests as well as performing calculations. The expanded use of electronic digital instrumentation and solid state power devices such as thyristors and power transducers, accelerates the development of more sophisticated equipment and measuring techniques for determining an electrical machine's overall performance.
In the prior art U.S. Pat. No. 3,052,117 motor testing apparatus is disclosed wherein a constant voltage source is required as input to the motor under test and its rotor speed is sensed using a DC tachometer generator. The DC tachometer generator produces an output signal which is linearly proportional to its rotational speed. This signal is used to obtain speed and torque characteristics of the motor under test. However, the prior art patent does not address the important motor performance characteristic known as "slip." Slip is the difference between designed synchronous rotor speed and the actual rotor speed of a motor and is usually expressed in percent of synchronous rotor speed or by a per unit representation. The technique used for measuring motor characteristics in the U.S. Pat. No. 3,052,117 patent will not produce an acceptable slip measurement because of the testing apparatus used. The DC tachometer generator used to provide an output voltage proportional to the rotor speed of the motor under test will cause a wide variation in slip characteristic due to a one percent error in the accuracy of the tachometer generator. The inability of a DC tachometer generator to maintain an acceptable output voltage proportional to speed is caused by the variation of brush contact due to uneven commutator surface and variation in brush tension.